The overall aim of this proposal is to understand how T cells survive Fas/tumor necrosis factor receptor (TNFR)-mediated apoptosis during the antigen-induced proliferation phase and how the same T cells become susceptible to the apoptosis at the end of the response. Antigen stimulation induces not only proliferation of T cells, but also the expression of molecules leading to cell death such as Fas, TNFR, and their ligands, Fas ligand (FasL) and TNFalpha. Binding of FasL and TNF-alpha to Fas and TNFR on activated T cells triggers a cell suicide program, which, however, occurs only at the end of the immune response. Under normal conditions, this ensures the elimination of unwanted, activated T cells, which, if left unchecked, can eventually lead to autoimmune disorders. It has generally been believed that in addition to Bcl-2 and Bcl-xL, two well- studied anti-death genes, the ability of the Fas+/TNFR+ T cells to proliferate at the early phase of immune response is primarily regulated by other as yet unidentified anti-death mechanisms. The applicant recently isolated a novel anti-death gene IEX-1L, that protects cells from Fas/TNFR-induced apoptosis. Its expression is rapidly induced by TNF-alpha stimulation through the activation of NF-kappaB/rel transcription factors, which renders the cells resistant to TNFR-induced cell death. The applicant hypothesizes that this may also be the mechanism by which T cells are protected from Fas/TNFR-induced cell death during T-cell activation. The applicant therefore proposes to further characterize IEX-1L expression in various in vitro models of T- cell activation to see whether a decrease in its expression is correlated to an increase in the sensitivity of T cells to Fas/TNFR- induced cell death. The applicant will also generate transgenic mice that express human IEX-1L in peripheral T cells under the control of H-2Kb promoter and dissect the protective role of IEX-1L in vivo. Transcription of the IEX-1L gene is controlled by mutiple transcription factors, which may be the key to regulate the sensitivity of T cells to apoptosis during various phases of the immune response. The applicant will determine these potential transcription factors in order to elucidate how differential expression of IEX-1L in T cells is achieved during immune response. Finally, they will identify the downstream targets that play an important role in the IEX-1L-mediated protection. These studies will help to elucidate the molecular basis of T-cell homeostasis and could potentially unravel novel targets for therapeutic intervention in lymphoproliferative disorders.